Shell and tube heat exchanger with central conduit

ABSTRACT

A shell and tube heat exchanger is disclosed having two axially spaced tube sheets that connect a plurality of straight tubes in fluid communication with heating fluid manifold chambers disposed at the opposite ends of the shell. A central conduit adapted for conducting heating fluid extends between and attaches at its opposite ends to the two tube sheets in a manner to vertically support the tube sheets and to relieve thermally induced compressive stresses that would otherwise be imposed upon the tubes.

United States Patent v[191 Romanos Apr.9,1974

SHELL AND TUBE HEAT EXCHANGER WITH CENTRAL CONDUIT Inventor: Nicholas D. Romanos, Chattanooga,

Tenn.

Filed: July 21, 1971 Appl. No.: 164,729

Related US. Application Data Continuation-in-part of Ser. No. 7,937, Jan. 30, 1970.

US. Cl. 165/158, 122/32 I Int. Cl...Q F28b 9/02 Field of Search 165/142, 158, 161; 122/32, 122/34 References Cited UNITED STATES PATENTS 2/1932 Smith l65/161 X Primary Examiner-Charles J. Myhre Assistant Examiner-Theophil W. Streule, Jr. Attorney, Agent, or Firm-John F. Carney ABSTRACT induced compressive stresses that would otherwise be imposed upon the tubes.

3 Claims, 3 Drawing Figures fee 84 SHELL AND TUBE HEAT EXCHANGER WITH CENTRAL CONDUIT This application is a continuation-in-part of U. S. Pat. No. application Ser. No. 7,937, filed Jan. 30, 1970.

BACKGROUND OF THE INVENTION Shell and tube heat exchangers are currently in use as vapor generators in systems employing reaction coolants as the heating medium. Typical examples are nuclear operated power plants wherein the coolant fluid of the reactor is used as the heating medium in the vapor generators. In these systems the vapor generator commonly comprises an elongated, cylindrical shell provided within its interior with a pair of axially spaced tube sheets that divide the shell into three axially spaced chambers. The two chambers at'the axial extremities of the shell define heating fluid collection manifolds while the intermediate chamber is substantially filled with a plurality of small diameter, thin walled tubes that are attached to the tube sheets. This latter chamber serves as the evaporator and superheating region of the unit. Fluid circulation between the vapor generator and the nuclear reactor is effected by means of large diameter external piping that connect the respective heating fluid chambers to the interior of the reactor.

As the size and capacity of nuclear power plants increase, deficiencies in the above described vapor generator design become evident. For example, as the length and diameter of the vapor generator shell are increased, the differential thermal expansion between the vapor generator and the reactor that must be accommodated by the interconnecting piping is greatly magnified. This, of course, imposes greater stresses on the piping especially at their points of connection with the vapor generator and the reactor. To counteract this problem it .has been necessary to provide those members of the system which are subject to these increased stresses, such as the external piping and the piping-toshell connections, with greater wall thicknesses. Alternatively, it has been contemplated to employ complex expansion-accommodating means to connect the ends of the piping to the reactor and vapor generators. Both of these solutions add considerably to the construction costs of the power plant and are, therefore, undesirable.

Another problem inherent in the above-described prior art vapor generator design is that the heat exchange tubes that fill the shell interior must tolerate high compressive loads. These loads are created by the fact that the tubes are subjected to considerably higher metal temperatures than the shell. Because the tubes are axially constrained due to their rigid connection to the shell through the tube sheet they cannot undergo thermal elongation attendant with the temperatures to which they are subjected. Instead, they will elongate only an amount commensurate with the amount of elongation experienced by the shell. Because the average metal temperatures of the shell material are considerably lower than those of the tubes, the'shell will have a tendency to expand considerably less than that of the tubes such that compressive stresses will be imposed upon the latter. Undue compressive loading of the tubes is undersirable due to the factthat, since the tubes are thin walled members, they are prone to failure by buckling.

an alternative, complex tube support structure can be employed to add columnar strength to the tubes but this too results in a considerable increase in fabrication costs.

It is to the solution of these problems, therefore, that the present invention is directed.

SUMMARY OF THE INVENTION According to the present invention there is provided a vapor generator of the shell and tube type that minimizes the above-mentioned problems. The structure includes an axially disposed, elongated conduit of enlarged diameter whose ends connect between the respective tube sheets. At one end the conduit is in fluid communication with the adjacent heating fluid manifold chamber. Its other end communicates, by means of a connector, with the exterior of the shell where external piping effects a connection with the reaction vessel for circulating heating fluid through the vapor generator.

The central conduit of the present invention serves as both a fluid conductor and a structural member. Its use eliminates the need for long external piping between the reactor and a vapor generator. Because the conduit is axisymmetrically loaded, moreover, it requires no special external support structure. Additionally, because the conduit is subjected to substantially the same fluids and thereby essentially the same metal temperatures as the heat exchange tubes, compressive loading of the tubes can be relieved by simply providing the conduit with sufficient thickness to compensate for the difference in total metal cross sectionalarea betwee the shell and the tubes.

For a better understanding of the invention, its operating advantages and the specific objects obtained by its use, reference should be made to the accompanying drawings and description which relate to a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 schematically illustrates a typical nuclear vapor supply system 10 of the prior art. It consists of a nuclear reactor 12 employing pressurized liquid as a coolant and a shell and tube type heat exchanger or vapor generator 14 through which the reactor coolant is circulated to transfer heat to a secondary fluid, such as water, whereby the latter is vaporized for use as the motive fluid of a prime mover (not shown). Piping, in-

cluding a supply line 16 and a return line 18, interconnects the reactor 12 and vapor generator 14 for the continuous circulation of coolant between the two apparatus. Circulation of the coolant is effected by means of a pump 20 disposed in the return line 18. The higher temperature coolant enters the vapor generator 14 at its top and is caused to flow downwardly through the heat exchange tubes contained therein. Secondary liquid enters the interior of the vapor generator 14 through an inlet 22 and flows about, and along, the tubes in indirect heat transfer relation with the flowing reactor coolant. The secondary liquid is thus vaporized and leaves the vapor generator 14 as saturated or superheated vapor through an outlet 24 from whence it is passed to a point of use.

It will be appreciated from an examination of the prior art system of FIG. 1 that various of the system components will encounter thermally induced stresses during operation of the plant. For example, the reactor coolant flowing through the supply line 16 is at a higher temperature than that conducted by the return line 18. The metal temperature experienced by the piping that forms the lines, and therefore the amount of elongation to which each is subjected, is different thus requiring special component support consideration. Similarly, metal temperature of the vapor generator shell will be a function of the temperature of the secondary fluid contained within the shell. The average metal temperature of the shell, and therefore the amount of its elongation will be considerably less than that of the combined elongation of the supply and return lines, 16 and 18. Thus, due to the fact that the wall of the vapor generator shell contains a greater cross sectional area than the wall of the supply and return line piping, the piping will experience compressive loading. Moreover, because the longitudinal axis of the piping is spaced a considerable distance from that of the vapor generator shell axis, a bending moment is produced between the components that are additive to the stresses already imposed upon the pipe.

Still another area of concern is the stress imposed upon the heat exchange tubes that comprise the tube bundle within the vapor generator. Because these tubes conduct reactor coolant which is at a considerably higher temperature than that of the secondary liquid contained within the vapor generator shell, and because the cross sectional area of the shell wall is greater than the total cross sectional area of the tube walls, the result will be to impose compressive stresses upon the tubes. As mentioned above, to so-load the tubes renders them prone to failure by buckling unless measures are taken, such as providing extensive tube support structure or additional tube thickness, both of which solutions add considerably to the fabrication cost of the system. 7

FIG. 2 illustrates a nuclear vapor supply system 30 incorporating a reactor 12 similar to that shown in FIG. 1 but with a vapor generator, indicated as 32, which is constructed according to the present invention. Piping, including a supply line 16 and a return line 18 containing a pump 20 serves to circulate coolant between the reactor and the vapor generator.

The vapor generator 32 is described in detail with regard to FIG. 3 and comprises a vertically elongated, cylindrical shell 40 having its ends closed by domeshaped closure heads 41 and 42 to define a substantially closed vessel. A pair of axially spaced upper and lower tube sheets 44 and 46 are provided adjacent the respective ends of the vessel. The tube sheets extend transversely of the shell axis and have their outer peripheral edges attached to the shell wall to divide the vessel into three axially spaced chambers. The upper and lower chambers, 48 and 50, are termed the heating fluid inlet and outlet chambers respectively and the intermediate chamber 52 is referred to as the evaporation chamber. A plurality of straight, thin walled tubes 54 disposed in a bundle extend through the evaporation chamber 52 from end to end thereof and in parallel relation to the vessel axis. The ends of the tubes 54 extend through, and are weldedly secured to, the respective tube sheets 44 and 46 to communicate with the heating fluid inlet and outlet chambers 48 and 50 for circulating coolant from the reactor 12 in indirect heat transfer relation with the fluid contained within the evaporator chamber 52.

As is common in vapor generators of the shell and tube type, the evaporator chamber 52 contains an elongated cylindrical baffle 56 that surrounds the tube bundle and is concentrically spaced from the shell wall to define an annular downcomer passage 58 and an internal riser passage 60. The lower end edge of the baffle 56 is spaced slightly above the upper surface of the lower tube sheet 46 to effect fluid communication between the downcomer and riser passages. A feedwater inlet nozzle 62 penetrates the wall of the shell 40 and serves to supply secondary liquid to the vapor generator 32. As shown, the feedwater inlet nozzle 62 communicates with the downcomer passage 58 within which the secondary liquid is caused to flow downwardly and thence into the riser passage 60. Within the riser passage, the secondary liquid passes upwardly in indirect heat transfer relation with the heated fluid flowing through the tubes 54. Circulation of the secondary liquid is maintianed by thermal siphonic action whereupon some, or all, of the liquid is transformed into vapor. The so-created vapor rises through the evaporation chamber 52 to the upper region thereof, indicated as the drying or superheating section 64, wherein it is further heated to dryness or even to a superheat temperature before exiting the vessel through a vapor outlet nozzle 66. To enhance heat transfer between the heating fluid and the flowing vapor a number of axially spaced, transversely extending baffle plates 68 may be disposed in the superheating section 64 to direct the vapor in cross-flow relation to the tubes 54.

The vapor generator 32 of the present invention is provided with an axially disposed conduit 70 of enlarged diameter that extends through the evaporation chamber 52. The opposite ends of the conduit are attached as by welding to the tube sheets 44 and 46. Central openings 76 and 78 are provided in the tube sheets at the points of attachment of the conduit ends by means of which the conduit 70, at its upper end, is caused to communicate with the upper coolant manifold chamber 48 and, at its lower end, with the exterior of the shell, by means of a cylindrical connector 80 which attaches to a nozzle 82 that penetrates the lower closure head 42. The nozzle 82 connects with the coolant supply line 16 and serves to supply reactor coolant to the interior of the vapor generator. A second nozzle 84 penetrates the lower closure head 42 in spaced relation from the nozzle 82. The nozzle 84 communicates with the lower coolant manifold chamber 50 and for discharging spent reactor coolant from the vapor generator connects with the reactor coolant return line 18.

High temperature reactor coolant is thus caused to flow from the supply line 16 seriatim through inlet nozzle 82, connector 80 and conduit 70 to the upper manifold chamber 48. From the chamber 48 the coolant is caused to flow downwardly through the tubes 54, emptying into the lower manifold chamber 50 and exiting the unit through the discharge nozzle 84 to the return line 18. It will be evident that the conduit 70, in conducting high temperature coolant through its interior and being exposed to secondary liquid on its exterior will thereby be subjected to substantially the same metal temperature as each of the tubes 54. Because the conduit is of substantially the same axial length as the tubes it will have a tendency to undergo thermal elongation and contraction at about the same rate as the tubes. More particularly, since the conduit 70 conducts high temperature coolant and attaches the tube sheets to which the tubes are attached, the combined metal cross section of the tubes 54 and conduit 70 is caused to be greater than the metal cross section of the shell 40. The shell 40 will thus be caused to elongate proportionately thereby reducing the axial strain in the tubes that creates the compressive stresses therein. Moreover, by providing the conduit with sufficient wall thickness the ratio of the respective cross-sectional areas can be made such that the compressive loads on the tubes can be optimized.

By providing a central conduit for conducting high temperature reactor coolant and which further serves as a structural support member in the manner taught herein gives rise to several advantageous features in addition to that described above. The conduit 70 of the invention, being disposed on the axis of the shell is subjected to axisymmetrical loading and thereby requires no external support structure. Additionally, by locating the portion of the coolant supply piping, which the conduit comprises, on the axis of the shell the bending moment between the external piping and the shell of the prior art arrangement is eliminated. Moreover, since the axial elongation of the conduit is accommodated within the confines of the shell enclosure the length of the required external piping and the thermal elongation experienced thereby are reduced to a minimum. This, in turn, enables the support arrangement for both the reactor and the vapor generator to be simplified. The entire length of the vapor generator can accordingly be located above the reactor thereby increasing the naturally induced pressure head available in the primary fluid circuit with a concomitant reduction in required pumping power.

Furthermore, the central conduit provides structural support to the tube sheets. ln attaching the tube sheets about their midpoint the extend of unsupported span is reduced resulting in a reduction in the amount of deflection which the tube sheets would otherwise experience. This feature further reduces the compressive loading on the tubes and further enables tube sheets to be employed that are of reduced thickness thereby reducing the fabrication costs of the plant.

A still further advantage derived from the use of the particular central conduit arrangement of the present invention is that the central conduit can be installed in the shell after all of the tubes of the tube bundle have been attached to the tube sheets. This enables an axial passageway to be provided through the tube bundle during assembly of the tubes thereby permitting workmens access to the interior region of the tube bundle for thereby facilitating installation and alignment of the tube supports and the tubes during the assembly procedures. In this way the time and effort attendant with the assembly is materially reduced.

It will be understood that various changes in the details, materials, and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. In a vapor generator including a shell defining a substantially closed vessel, a pair of axially spaced tube sheets attached at their periphery to the wall of said shell and dividing said vessel into inlet and outlet manifold chambers and an intermediate evaporation chamber, a plurality of tubes extending through said evaporation chamber and having their opposite ends attached to each of said tube sheets and being in fluid communication with the respective manifold chambers, means for admitting low temperature vaporizable liquid to said evaporation chamber and means for circulating high temperature heating fluid through said manifold chambers and said tubes, the differential cross sectional areas and material temperatures between said shell and said tubes being such as to impart a compressive loading upon said tubes, the improvement comprising means to reduce the compressive loading upon said tubes including a conduit for conducting said heating fluid to said inlet manifold chamber, said conduit extending substantially parallel to said tubes through said evaporation chamber and being attached at spaced locations to each of said tube sheets.

2. The organization of claim 1 in which said conduit has an internal diameter greater than that of said tubes.

than that of said shell. 

1. In a vapor generator including a shell defining a substantially closed vessel, a pair of axially spaced tube sheets attached at their periphery to the wall of said shell and dividing said vessel into inlet and outlet manifold chambers and an intermediate evaporation chamber, a plurality of tubes extending through said evaporation chamber and having their opposite ends attached to each of said tube sheets and being in fluid communication with the respective manifold chambers, means for admitting low temperature vaporizable liquid to said evaporation chamber and means for circulating high temperature heating fluid through said manifold chambers and said tubes, the differential cross sectional areas and material temperatures between said shell and said tubes being such as to impart a compressive loading upon said tubes, the improvement comprising means to reduce the compressive loading upon said tubes including a conduit for conducting said heating fluid to said inlet manifold chamber, said conduit extending substantially parallel to said tubes through said evaporation chamber and being attached at spaced locations to each of said tube sheets.
 2. The organization of claim 1 in which said conduit has an internal diameter greater than that of said tubes.
 3. The organization of claim 1 in which the combined metal cross section of the tubes and conduit is not less than that of said shell. 